[801] The Treatise on the Resurrection shows that the transfiguration was
not a vision but a reality

This treatise is thought to have been written in the second century (The Nag
Hammadi Library James Robinson). It appears to us to be true. Here is what is
says about the transfiguration...

For if you remember reading in the Gospel that Elijah appeared and Moses with
him, do not think the resurrection is an illusion, but it is truth! Indeed it is
more fitting to say that the world is an illusion, rather than the resurrection
which came into being through our Lord and Saviour Jesus Christ (From The
Treatise on the Resurrection)

So the author plainly believes as we do that the 3 apostles Peter James and
John say the risen Elijah and the risen Moses.